To figure out hobbit/human age equivalents, divide a hobbit's age by 3, and then double the result. Thus, at 33, hobbits officially become adults when they reach the physical and mental maturity level of a 22 year old human being. Pippin embarking on his adventure at 27, not quite an adult but close enough, comes out to the hobbit equivalent of a human 18. At the time that Tolkien wrote that, most western countries considered an 18 year old person old enough to serve in the military but not old enough to vote. Bilbo departs on his adventure at a "middle-aged" 50, which is the equivalent of being in his thirties.

Frodo acquired the ring just when he came of age, and stopped aging at that point--so he really was supposed to look too young for his years. In fact, in the books, his unnaturally youthful appearance and behavior generated much comment in the Shire, along with concern that in some way it would have to be "paid for". My website http://www.dreamdeer.grailmedia.com offers fanfic, and message-boards regarding intentional community or faerie exploration.